1. Field of the Invention
The invention relates to a reciprocating piston type compressor for use in an automobile air conditioning system.
2. Description of the Related Art
A reciprocating piston variable displacement type refrigerant compressor for use in an automobile air conditioning system is known in the art. Such a compressor comprises a cylinder block including a plurality of parallel cylinder bores arranged around an axial drive shaft, and pistons slidably provided within the cylinder bores for reciprocating between the top dead center and the bottom dead center. The compressor is provided with a drive mechanism for reciprocating the pistons.
The drive mechanism comprises an axially extending drive shaft which is operatively connected to an automobile engine, and a swash plate which is provided within a crank chamber and is mounted on the drive shaft by a tilting mechanism for changing the angle of the swash plate relative to the drive shaft. The swash plate is engaged with the pistons through shoes mounted on the respective pistons.
The crank chamber is fluidly communicated with a high pressure source, for example the discharge pressure of the compressor, through a solenoid type displacement control valve. A computer controls a solenoid driver connected to the solenoid valve, according to a cooling load demand.
When the cooling load demand is relatively high, the solenoid driver increases the electric current supplied to the solenoid valve so that the solenoid valve operates to decrease the degree of opening, which results in decreasing the pressure in the crank chamber. Decrease in the crank chamber pressure also decreases the differential pressure across the pistons, that is the differential pressure of the refrigerant gas between the crank chamber and a suction chamber so that the swash plate moves relative to the drive shaft to increase the stroke of the piston and the displacement of the compressor.
Recently, automobile engines have been produced which can operate at a high rotational speed. The higher the rotational speed of the automobile engine, the higher the rotational speed of the compressor. Operation of a compressor at a relatively high rotational speed and displacement makes the load of the compressor high.
Conventionally, in order to solve this problem, a computer provides a protective control for a compressor at high rotational speed of an automobile engine which is the power source for the compressor. A speed sensor detects the rotational speed of the automobile engine and the computer compares the detected speed with a reference speed. The computer further compares the current supplied to the solenoid valve with a reference current value. The computer will generate a signal to the solenoid driver to increase the electrical current to the solenoid valve when both the compared values are higher than the reference values. Thus, the displacement of the compressor decreases to reduce to the load of the compressor.
However, the above-mentioned prior art includes the following problems.
The protective control by a computer makes the program for controlling the compressor complex and increases the necessary memory capacity.
Generally, an overload is realized in a compressor when the compressor operates at high rotational speed and high displacement. However, some conditions prevent overload of the compressor even if a compressor operates at high rotational speed and high displacement. According to the prior art, the displacement of a compressor always decreases so that the compressor does not satisfy the cooling load when the compressor operates at high rotational speed and high displacement even when the compressor is not overloaded. On the other hand, according to the prior art, a compressor cannot reduce its displacement, if some condition results in overload, when the compressor operates at middle or low speed and displacement since in such a case, the computer does not provide the protective control.
Further, according to the prior art, when a driver circuit for the solenoid valve breaks down due to malfunction of a power source for the driver or short circuit in the driver, a current higher than the rated current may be supplied to the solenoid valve so that overheat or damage in the solenoid valve results. However, the prior art cannot prevent such an excess current since the computer controls the current to the solenoid based on the rotational speed and displacement of the computer.